This invention relates to a process for reducing the distortion of multi-layered composite structures, particularly such structures which distort upon changes in temperature, at least one layer of which exhibits substantial creep under stress at an elevated temperature.
Composite structures in which plastics are laminated to other plastics or to metals are in common usage in a variety of applications. These structures have found such widespread usage mainly because they possess a combination of properties which can not be found in any single material. Many of these materials suffer from the significant disadvantage that they tend to distort when subjected to temperature changes. This behavior is similar to that of a bimetallic laminate such as used in thermostats. Since the composite structure is generally formed at an elevated temperature and thereafter cooled, it is quite often distorted when it reaches the temperature at which it will be used.
This distortion causes dimensional changes in the composite structure which are particularly severe if the area of the structure is large. In severe cases, this distortion can cause the layers to delaminate. The distortion also makes it difficult to fit the structure to other parts in preparing an assembly. The distortion and build-up of stresses in the composite can also lead to cracking and general weakness in the structure.
This problem is of particular significance in appliance industry, particularly the refrigerator and freezer industry. The walls of refrigerators and freezers commonly comprise an innermost layer of a thermoplastic resin, a central layer of a polyurethane insulating foam and an outer layer of steel or other metal. These walls are generally prepared by forming an empty shell of the inner and outer layers, and forming the polyurethane in place. The heat of reaction in the polyurethane foams heats the entire wall, which then distorts on cooling. The distortion in the walls varies its dimensions making it difficult to mount shelves and other accessories. Furthermore, cracking in corners of the inner wall leads to a high rate of rejection of the walls.
Due to the aforedescribed problems associated with distortion of composite structures, it would be desirable to provide a process whereby said distortion is reduced.